Matter and particles, Year 8

Jitter about randomly

HintThey never keep still or keep to a straight path.

The whyEach speck shows as a bright dot that darts a tiny distance one way, then another, with no pattern. This is called Brownian motion.

It is made of moving particles too small to see

HintSomething invisible must be doing the pushing.

The whyThe specks cannot push themselves about. Their jittering was the first direct evidence that the air is made of fast-moving particles.

The collisions come unevenly from all sides

HintNo two instants bring an identical set of knocks.

The whyAir particles hit the speck from every direction. By chance the hits never quite balance, and the imbalance keeps changing.

No — the air particles never stop moving

HintAsk whether the cause has gone away.

The whyAs long as the air is at any ordinary temperature, its particles keep moving and keep colliding with the specks.

The metal lid expands and loosens

HintThink about what heating does to the size of a solid.

The whyHeated particles vibrate more and push each other slightly further apart. The lid grows a little wider than the glass and loses its grip.

No — they move further apart

HintA hot iron bar is longer, yet it has no extra atoms.

The whyThe particles vibrate more strongly and so take up more room each. The solid grows because the spacing grows.

To give the steel room to expand on hot days

HintThink about what a long strip of metal does on a hot day.

The whyA long bridge can grow by several centimetres between winter and summer. The gaps, called expansion joints, let it do so safely.

Neither — they have the same mass

HintRead the two amounts carefully.

The whyA kilogram is a kilogram. The feathers take up far more space because they are much less dense, but the two masses are equal.

Iron is denser: more mass is packed into the same space

HintCompare how much matter fits in each cubic centimetre.

The whyDensity compares mass with volume. In iron the particles are heavier and more closely packed, so each cubic centimetre has more mass.

Its particles are much further apart

HintThink about the amount of empty space there is.

The whyThe particles are identical in both states, but in the gas they are spread through a far larger volume, so each cubic centimetre holds very little mass.